1. Field of the Invention
An object of the invention is a temperature threshold detection circuit which can be used in all fields where temperature has to be monitored. However, the invention can be more especially used in the field of integrated circuits and, therein, in that of memories.
2. Description of the Prior Art
It is known that using electronic integrated circuits cause them to be heated up through the passage of different current pulses which excite them, especially during programming operations. For, especially with non-volatile memories provided with memory cells (of the EPROM or EEPROM type) with floating gate transistors, the current or voltage pulses should be greater to force the electrical charges to migrate into the floating gates of the transistors of the memory cells at the instant when these cells are programmed. If the programming rate is too high, it may result in an irreversible deterioration of the integrated circuit. To overcome this drawback, there is a known way to limit the programming rate according to the cooling capacity of the integrated circuit to be programmed. This drawback, however, limits the use of the memories for which the memory cell is provided with floating gate transistors, such as the random access memory of a microprocessor.
Furthermore, it can be assumed that dishonest individuals will try and use the programming aberrations that might result from artificial external heating of an integrated circuit of this type before it is programmed. These dishonest individuals might also be tempted, during this programming operation, to keep these circuits at temperatures which are below the irreversible destruction threshold but above a normal operating limit threshold of the circuit. During a subsequent cooling, the content of the information stored in a circuit of this type could be different from the information introduced therein during the programming operation. This different content could enable the use of a memory card, fraudulently modified in this way, for dishonest purposes. An operation in reverse can also be envisaged. If comprehensively heated, a card provided with a regularly programmed memory could temporarily have a modified content of information and would there would be the risk that this would enable operations to which the card should normally prevent access. This latter type of fraud would be all the more subtle as it is incapable of being detected. For, when checked in the cold state, the card would once again appear to conform to requirements.
There are known temperature detectors based on integrated circuits essentially comprising a directly biased MOS type transistor with a conduction threshold known to vary with the temperature. Typically, for silicon transistors, this conduction threshold varies between 0.3 volts and 0.5 volts for a variation in temperature of about 100.degree. C. at the habitual temperatures of use. However, this temperature detector has the drawback of not being very sensitive: a very low voltage variation corresponds to a wide range in temperature. Furthermore, while this voltage variation does not depend on the geometry of the transistor made, it depends to a great extent on the concentrations of impurities effectively implanted in the source and drain regions and in the transistor pads thus made to act as temperature detectors. For, owing to natural variations in manufacturing conditions, these temperature detectors become unreliable in addition to lacking sensitivity. Their detection threshold varies excessively from one detector to another. In practice, these approaches are rejected and replaced by precautionary set values tending to restrict the use of integrated circuits of this type, thus reducing their use as random access memories: this, of course, has no effect on the intentions of dishonest persons. An object of this invention is to overcome these drawbacks by proposing the use of another physical phenomenon. For, it has been realized that the saturation current of a reverse biased transistor changes in an appreciable manner as a function of the temperature. It can even be shown that this current can get doubled while the temperature varies by only a few degrees C. Furthermore, in an improved embodiment where two transistors are used mounted in series and both reverse biased, it can be shown that the calibrated thresholds of detection are less sensitive to manufacturing variations and are also less sensitive to the conditions of use, i.e. when the supply voltage diverges from a rated value Vcc.